Significance Statement

The commercial lithium ion batteries based on graphite anode is known for its low rate performance. Titanium oxide appears to be a promising building block for electrodes used in lithium-ion storage owing to its superior charge storage capability, environmental benignity and low cost. Unfortunately, titanium oxide has poor ionic and electronic conductivity as well as substantially low lithium-storage capacity. This poses numerous challenges towards achieving high performance devices.

Several researchers have devoted their efforts explore various polymorphs, including titanium oxide-bronze, rutile and anatase phases. Titanium oxide-bronze exhibit faster lithiation/delithiation kinematics and higher capacity as compared to rutile and anatese. However, it preparation requires harsh and complex conditions making it less attractive for large-scale preparation.

Another promising approach is through structure and morphology control. Including composite architectures based on low-dimensional titanium oxide; Core-shell morphologies such as carbon coated titanium oxide nanoparticles and cable like titanium oxide carbon nanotubes. The low-dimensional titanium oxide offers shortened ion diffusion length and the conductive inclusions enable effective electron transport granting these composites high electrochemical storage performance. However, the titanium oxide nanocrystals are assembled around the conductive moieties that may disassemble easily from the conductive networks and lead to rapid capacity fading. More importantly, previous works with low dimensional crystals often demonstrate very low tapping density (<0.2 g cm3) which would not be feasible in real applications.

Researchers led by professors Yunfeng Lu and Hexing Li at university of California Los Angeles and in collaboration with scientists at Shanghai University of Electric Power and Shanghai Normal University, developed a nanocomposite architecture prepared by in-situ growth mesoporous single-crystal-like titanium oxide particles threaded through by carbon nanotubes. Their work is now published in Nano Energy.

Assembled from sub-10 nm anatase nanocrystals, single-crystal secondary particles in micrometer scale with mesoporous features and threaded by carbon nanotubes (CNTs) was synthesized by one step sovolthermal method. No further high temperature heating required.

The electrochemical performance of the composite was characterized by using coin-type cells with loading of 2-3 mg/cm2. From the galvanostatic charge/discharge curves, the composite achieved 260 mAh g-1 at 0.5C which is among the highest capacities of the reported TiO2 anode. A relatively short voltage plateau (phase charge) with a long slopping “tail” (200 mAh g-1) indicated a high surface charge contribution. Even at an extremely high rate of 30C, it still delivered 120 mAh g-1. The researchers also investigated the composite performance at extended voltage window from 0.005-2.7V. Such lower voltage was often avoided due to formation of solid electrolyte interface (SEI) and further lithium insertion would challenge the robustness of the active materials. With sub-10nm building nanocrystals, the composite achieved 440 mAh g-1 at 50mA g-1. As seen from the cyclic voltammetry diagram, such “extra” capacity below 1V main came from surface contribution. Above 1V to 2.7V, the composite still maintained the reversible anatase phase confirmed by the in-situ X-Ray Diffraction. Overall, high rate charge discharge at 2000 mA g-1 for 1000 cycles was demonstrated with negligible capacity fading. The scanning electron microscopy after cycling confirmed the structure was maintained

By threading single crystal -like titanium oxide mesocrystals with carbon nanotubes, the research team was able to realize high rate anode composite for lithium ion batteries. The composites with iso-orientated primary nanocrystals has successfully enhanced the rate and cycling performance. Importantly, the tapping density (1.12 g cm3) is much closer to the real applications which was often ignored in nano-scale engineering. This method might bring some guidelines for the functional materials and device towards highly efficient energy storage systems.

Tapping-Density-ComparisonSynthesis-SchemeDesign-Stratagies

About the author

Yiting Peng obtained her Ph.D. degree under the co-supervision of Professor Hongbin Geng in Harbin institute of technology and Professor Yunfeng Lu in University of California, Los Angeles at 2013. She is now a Lecturer at Shanghai University of Electric power.

About the author

Zaiyuan Le received his BASc degree in Materials Science and Engineering from University of Toronto (Canada) in 2012. He is currently a Ph.D. candidate under supervision of Prof. Yunfeng Lu in Chemical and Biomolecular Engineering at University of California, Los Angeles.

His research interests lie in titanium oxide based lithium and sodium based energy storage, including rechargeable batteries and hybrid capacitors.

About the author

Meicheng Wen received his M.S. degree in physical chemistry from Shanghai Normal University under supervision of Professor Hexing Li and Associate Professor Dieqing Zhang in 2013 and his Ph.D. degree in engineering from Osaka University under the supervision of Professor Hiromi Yamashita in 2016. He is currently a specially appointed Assistant Professor in the Division of Materials and Manufacturing Science at Osaka University.

About the author

Dieqing Zhangreceived her PhD (2010) in environmental chemistry from The Chinese University of Hong Kong under Prof. Jimmy C. Yu. Currently is an Associate Professor in Department of Chemistry at Shanghai Normal University. She is now engaged in the design and fabrication of novel and efficient photocatalysts for nitric oxide oxidation/reduction, hydrogen production and CO2 reduction, etc.

About the author

Zheng Chen received his Ph.D. at UCLA in 2012 under the supervision of Prof. Yunfeng Lu in the Department of Chemical and Biomolecular Engineering. From 2013-2016, he was a postdoctoral associate working with Prof. Zhenan Bao in Chemical Engineering and Prof. Yi Cui in Materials Science and Engineering at Stanford University.

Currently as Assistant Professor at Department of NanoEngineering at UCSD, his research focuses on functional polymers, nanostructured materials and hybrids for applications in electrochemical energy, flexible devices and sustainable environment.

About the author

Hao Bin Wu received his BS degree in chemistry from Fudan University (China) in 2010. He obtained his Ph.D. degree in materials science from Nanyang Technological University (Singapore) under the supervision of Professor Xiong Wen (David) Lou in 2015.

Currently he works with Professor Yunfeng Lu as a Postdoctoral Scholar at University of California, Los Angeles. His research interests focus on synthesis and applications of nanostructured and hybrid materials for electrochemical energy storage and conversion, including rechargeable batteries, electrochemical capacitors and electrocatalysis.

About the author

Prof. Hexing Li received his doctor degree from Fudan University. Now, he is working as the president of Shanghai University of Electric Power and the director of Chinese Education Ministry Key Laboratory and International Joint Laboratory on Resource Chemistry, as well as an Associated Editor of Appl. Catal. B Environ.

His research interest is photocatalysis for environmental cleaning and thermocatalysis for green chemistry. Up to now, more than 395 papers and 3 monographs have been published. His H-index is 59.

About the author

Yunfeng Lu obtained his Ph.D. degree under the supervision of Professor C. Jeffrey Brinker in University of New Mexico at 1998. He became a Brown Chair Professor at Tulane University in 2005 and now he is a Professor at University of California, Los Angeles.